Microstructural and mechanical study of zirconia-hydroxyapatite (ZH) composite ceramics for biomedical applications

Hydroxyapatite is the mineral component of natural hard tissues and, as such, it has been studied extensively as a candidate biomaterial for its use in prosthetic applications. However, the main weakness of this material lies in its poor mechanical strength which makes it unsuitable for load-bearing applications. On the other hand, partially stabilized zirconia has been widely studied on account of its high strength and fracture toughness, and good biocompatibility. Therefore, it is believed that the addition of a particulate zirconia phase to a hydroxyapatite one may lead to an improvement of the mechanical properties of this kind of composite and will not affect its biocompatibility. In this study, two series of zirconia- hydroxyapatite composites, Z4H6 and Z6H4 with 40 and 60 vol.% of zirconia content respectively, were prepared by powder uniaxial pressing at 700 MPa and sintering in air at 1200–1500°C for 3 h. The sintering behavior, microstructural characteristics and mechanical properties were evaluated. Variation of average grain size for the zirconia and hydroxyapatite phases with sintering temperature was observed. Relative densities ranging from 89 to 91% of the theoretical values were reached for the Z6H4 ceramic series and for the Z4H6 sample sintered at 1400°C, although microcracks were present in all specimens sintered at 1400 and 1500°C. Values of ultimate compressive strength, Youngʹs modulus, micro-Vickers hardness and Poissonʹs ratio near to those for human cortical bone and human tooth (dentine and enamel) were found for almost all samples investigated suggesting that these materials present potential applications as structural implants.